Introduction An unintended consequence of indoor smoking restrictions is the relocation of smoking to building entrances, where non-smokers may be exposed to secondhand smoke, and smoke from outdoor areas may drift through entrances, exposing people inside. Tobacco smoke has been linked to numerous health effects in non-smokers and there is no safe level of secondhand smoke (SHS) exposure. This paper presents data on levels of tobacco smoke inside and outside entrances to office buildings.

Methods Real-time air quality monitors were used to simultaneously measure respirable particulate matter (PM_2.5; air pollutant particles with a diameter of 2.5 μm or less) as a marker for tobacco smoke, outside and inside 28 entrances to office buildings in downtown Toronto, Ontario, in May and June 2008. Measurements were taken when smoking was and was not present within 9 m of entrances. Background levels of PM_2.5 were also measured for each session. A mixed model analysis was used to estimate levels of PM_2.5, taking into account repeated measurement errors.

Results Peak levels (10 s averages) of PM_2.5 were as high as 496 μg/m³ when smoking was present. Mixed model analysis shows that the average outdoor PM_2.5 with smoking was significantly higher than the background level (p<0.0001), and significantly and positively associated with the number of lit cigarettes (p<0.0001). The average level of PM_2.5 with ≥5 lit cigarettes was 2.5 times greater than the average background level.

Conclusions These findings support smoke-free policies at entrances to buildings to protect non-smokers from exposure to tobacco smoke.